An optical gyroscope measures the interference pattern generated by two light beams, traveling in opposite directions within a mirrored ring (laser or “ring laser” gyroscopes) or fiber loop (fiber optic gyroscopes), in order to detect very small changes in motion. Optical gyroscopes are based on a principle called the “Sagnac effect” discovered at the beginning of the 20th century. Optical gyroscopes have a variety of applications, but are particularly well suited for use in guidance, navigation, and control systems such as those of aircrafts and spacecrafts. An example of an optical gyroscope can be found in U.S. Pat. No. 4,545,682, incorporated herein by reference in its entirety.
There are several different types of fiber optic gyroscopes including at least interferometer fiber optic gyroscopes (IFOGs) and resonator fiber optic gyroscopes (RFOGs). IFOGs are phase sensitive devices with the Sagnac effect generating an optical phase difference between two counter-propagating light beams/waves in a rotating fiber coil. RFOGs are frequency sensitive devices with the Sagnac effect generating a frequency difference between two resonant beams in a ring fiber cavity locked to resonant clockwise and counterclockwise frequencies.
IFOGs typically operate in either an open loop or a closed loop configuration. In the closed loop configuration, a phase shift equal in magnitude but opposite in sign to the phase shift generated by the Sagnac effect is generated, and the magnitude of the generated phase shift is then determined from the apparatus generating it.
To provide increased sensitivity at low angular rotation rates, an IFOG can introduce phase modulation between the counter-propagating light beams. An explanation of such bias modulation can be found in U.S. patent application Ser. No. 10/078,182 (Pub. No. US 2003/0169428), incorporated herein by reference in its entirety
In many instances an IFOG will comprise an optical receiver or other mechanism for converting the light output (which is a combination of the two counter-propagating beams) by a coil assembly to an analog or digital signal. As the term is used herein, the “optical power” of the IFOG is a measurement of the power of the light being output by the coil assembly and fed into the optical receiver.
FIG. 1 provides a schematic of an exemplary IFOG. In FIG. 1, an IFOG 1 includes a fiber optic loop 3, an optical phase modulator (OPM) 5, a coupler 7, a light source 9, a photo detector 11, and a control unit 13. A light signal from the light source 9 is split into clockwise (cw) and counter-clockwise (ccw) signals that pass through the fiber optic loop 3 and the OPM 5 before being re-combined in the coupler 7 and directed to the photodetector 11. The output of the photodector 11 is provided to the control unit 13 which controls the OPM 5 and, optionally, the light source 9.
FIG. 2 illustrates the paths followed by the cw and ccw signals of the IFOG 1. As shown, the cw signal travels from point P1 at the coupler 7, through point P3 on the loop 3, through the OPM 5, and back into the coupler 7 at point P2. The ccw signal travels from point P2 at the coupler 7, through the OPM 5, through point P3 on the loop 3, and back into the coupler 7 at point P1.
FIG. 3 illustrates a method of controlling the OPM 5 where the OPM 5 introduces a phase difference of forty five degrees during each interval T0-T3, with the sign of the phase difference introduced alternating between time intervals. In some instances, the time intervals will correspond to the transit time of light passing through the loop 3 such that there is a ninety degree phase difference between the cw and ccw signals as they re-enter the coupler 7.
FIG. 4 provides a schematic of an alternate form of the IFOG 1 of FIG. 1. In FIG. 4, the IFOG 1′ has all the components of the IFOG 1 of FIG. 1, except that it includes two OPMs 5A and 5B which are both part of a single integrated optical chip 5C in place of the OPM 5 of FIG. 1. The OPMs 5A and 5B are controlled (via control of the IOC 5C) to each provide half of the phase shift seen by the cw and ccw signals.
FIG. 5 illustrates the paths followed by the cw and ccw signals of the IFOG 1′. As shown, the cw signal travels from point P1 at the coupler 7, through OPM 5A, through the point P3 on the loop 3, through the OPM 5B, and back into the coupler 7 at point P2. The ccw signal travels from point P2 at the coupler 7, through the OPM 5B, through point P3 on the loop 3, through the OPM 5A, and back into the coupler 7 at point P1.
FIG. 6 illustrates a method of controlling the OPMs 5A and 5B where the OPMs 5A and 5B each introduce a phase difference of 22.5 degrees during each interval T0-T3, with the sign of the phase difference introduced alternating between time intervals, and the phase difference introduced by the OPM 5A being opposite in sign than that introduced by the OPM 5B. The time intervals T0-T3 each correspond to the transit time of light passing through the loop 3 such that the OPMs 5A and 5B add a total of 45 degrees (2 times 22.5) to each of the cw and ccw signals with the result that there is a ninety degree phase difference between the cw and ccw signals as they re-enter the coupler 7.
It should be noted that for both the IFOG 1 and the IFOG 1′, the OPMs (5, 5A, and 5B) are used to control the phase difference between the cw and ccw signals as they are recombined in the coupler 7 before being provided to the photodetector 11, with the signal at point P4 entering/detected by the photodetector 11 being the interference signal resulting from combining the cw and ccw signals. In both the method of FIG. 3 and the method of FIG. 6, the OPMs are controlled to introduce a phase difference of ninety degrees between the cw and ccw signals. As such, both methods can be described in regard to how that phase difference is controlled as shown in FIG. 7 which indicates that the phase difference between the cw and ccw signals during the intervals T0-T4 is always ninety degrees.
As will be discussed, exemplary embodiments of the present invention utilize alternative methods of control than those illustrated in FIGS. 3 and 6-8, and control of OPMs to produce patterns of phase differences will be described. As such, it is useful to generalize the pattern of FIG. 7 such that the pattern of FIG. 7 is seen as a repeated first phase difference D1. D1, which need not be ninety degrees, is introduced at each interval during a sequence of intervals T0-T4. As such, FIG. 8 illustrates the same control pattern as does FIG. 7, but without explicitly identifying a value for D1.